Moped clutch – automatic and simple

How a moped clutch works

Not all vintage mopeds are fitted with a variator, but they all have an automatic clutch. As on a motorcycle with a manual gearbox, its job is to disconnect or connect the engine and the transmission. The key difference is the centrifugal clutch, which engages automatically once a certain engine speed is reached.

Unlike a modern scooter, the clutch can work in two different ways:

• On mopeds with pedals, the clutch is driven directly by the crankshaft. The clutch works by centrifugal force and has clutch shoes that are held closed outwards by springs (which drive the clutch bell), but also a centre plate that is pushed out by balls. The centre plate mainly affects the take-off point. The shoes keep the clutch firmly engaged in operation.
  As rpm increases, rotational speed rises, centrifugal force increases and the shoes open to bite into the bell, which then directly drives the V-belt. At idle, the belt therefore remains stationary.
• On mopeds with kickstarter, the clutch sits on the rear pulley. Also designed as a centrifugal clutch, it is driven by the V-belt and opens as engine speed rises. It then drives the bell, which in turn drives the final drive gear. Unlike the system with the clutch at the front, the drive belt also rotates at idle here.

In both cases, no direct clutch action is needed to get things going.

ATTENTION: On some German models (Simson, Kreidler, ...), which are listed as mopeds, the clutch is not automatic. Here the operating principle is similar to a motorcycle clutch.

Moped clutch and engine tuning

As with any two-wheeler, the clutch needs to be adapted when power increases. On mopeds, this is not only about reliability, but also about the engagement point. A centrifugal clutch can be set up very precisely.
Since tuned engines usually deliver their peak performance at higher RPMs, it is especially beneficial to delay the engagement point. This ensures a snappier start and improved acceleration through the rev range. Depending on the system (clutch on the crankshaft or on the pulley), variation is possible via the balls, the star spring of the clutch disc, the clutch springs, or the weight of the clutch shoes.

Recommendations:

Depending on the tuning level (setup), it is not necessarily required to replace the complete clutch. However, setting it up is usually essential.

• Switching from the original exhaust to a performance exhaust? It can be worth trying to make the clutch lighter by removing balls or fitting several star springs (on the disc). Harder clutch springs are also available.
  Example: Malossi spring set
• Upgrading the moped with a more powerful cylinder? Then a reinforced and lighter clutch should be considered. It is more durable and engages at higher rpm.
  → Example: reinforced Motoforce disc, lightened Motoforce clutch shoes, Doppler sport clutch
• Building a proper race engine with a very peaky power curve (high power in a narrow rpm range): A reinforced and at the same time lighter clutch is needed here, ideally as a complete kit. In this case, it is advisable to switch to a system on the rear pulley to reduce the weight on the crankshaft and make setup easier.
  → Example: Doppler ER2 system

In addition, another clutch element is important: the clutch bell. The more powerful the engine, the hotter the clutch can get. The bell should therefore be matched accordingly, and a ventilated model should be chosen that fits the clutch.

Identifying faults and incorrect settings

Before discussing defects: on a stock engine, clutches are extremely long-lasting and almost never wear out. On tuned engines, they wear faster, especially if they are set up for very aggressive starts.

The main symptoms of wear are: the clutch slips, engagement is difficult, or it no longer engages at the right point (springs have weakened or stretched).

The main symptoms of incorrect setup are:

• The engine revs very high before the clutch bites, and take-off is very jerky: springs too stiff, or balls and shoes too light.
• The clutch engages while rpm is still too low, take-off is sluggish: springs too soft or stretched too much.

Moped clutches in our shop:

In our online shop, everything needed for servicing, setting up and improving the moped clutch can be found. Complete clutch kits are available, both in OEM quality and reinforced, as well as all individual components: springs, discs, bells, shoes, accessories and more. Parts from well-known manufacturers such as Malossi, Polini, Doppler, Motoforce are available, and of course for popular models such as MBK 51, Magnum Racing, Peugeot 103 SP / SPX / Vogue / RCX etc.